At first, the rod of a track belt of snowmobiles was made of steel material. However, the snowmobile has been widely and generally used as a vehicle in cold areas and extensively used for leisure. In accordance with these requirements, and also to meet the demand on the market to save the energy and to enhance performance, fiber reinforced plastics are now used.
Leisure use of snowmobiles has been recently emphasized more and more and larger and higher performance snowmobiles have been used. Accordingly, the conventional high velocity of snowmobiles in the range of 120 to 150 km/hr has to been increased to in the range of 160 to 200 km/hr.
As a result, for rods using conventional fiber reinforced plastics, there is a great fear that the rod with break when the snowmobile runs. It has been found that rods of conventional fiber reinforced plastics are not durable and, in addition experience seriously deteriorated safety.
According to analyses, it has been found that the reason for the above described problems is that the temperature of the fiber reinforced plastics is elevated up to a high temperature in the range of 180 to 200.degree. C. due to mechanical friction caused by continuous high velocity running of the snowmobile. Heat generated by such running is retained by the rods. The temperature largely exceeds the glass transition point, typically about 130.degree. C., of the fiber reinforced plastics.
Accordingly, it is necessary to provide a material for the rods, which, of course, has high bending fatigue characteristics in an environment including temperatures from room temperature to 80.degree. C. experienced during routine running at less than 100 km/hr while maintaining high heat resistance at temperatures above 200.degree. C. The rod material should also be durable against repeated bending loads randomly applied to the belt during running on convex and concave surfaces. In other words, the material should exhibit bending fatigue characteristics.
Almost all of the practical fiber reinforced plastics that have been used in the rods are made by plastic drawing and molding using unsaturated polyester resin, a vinyl ester resin or an epoxy system resin in a matrix.
However, although unsaturated polyester resin and vinyl ester resin are easy to handle and experience superior moldability, their heat resistance is limited to up to 150.degree. C. They also exhibit generally inferior bondability to epoxy system resins. Accordingly, the unsaturated polyester resin and vinyl ester resin have a problem with bending fatigue characteristics.
On the other hand, the epoxy system resin is superior in bondability and has a wide variety of kinds. Also, epoxy resins have superior heat resistance. However, it is well known that, if heat durability is regarded as an important factor, the resin becomes hard and fragile, resulting in remarkable degradation in bending fatigue characteristics.
Also, a method is well known in which the hardness or fragility of the epoxy resin is improved to enhance tenacity to enhance the bending fatigue characteristics by adding a rubber modified resin or a flexibility enhancing agent. However, in this case, the heat resistance is degraded. This does not meet the required performance.